This invention relates to an automatic focusing device which automatically adjusts a camera photographing lens to a focus position by an electrical signal developed at the detection of the focus condition.
In the conventional automatic focusing device, the principle of a double image coincidence type range finder having a pair of photoelectric cells has been used, wherein the photographing lens is fixed at the focus signal given when the images of a fixed mirror and of a movable mirror interlocked with the photographing lense coincide. This method has a disadvantage in that photographing is performed with the photographing lense displaced to the infinite range or to the nearest range setting if the focus signal is not given when the focus detection is impossible because the image signal of the movable mirror is given after the photographing lens started moving driven by some power device. For instance, in the conventional method as shown in FIG. 1, the lens tube (k) released by depressing the release button (not shown) starts moving pulled by a spring (1) in the direction shown by an arrow. A movable mirror (b) interlocked with the lense tube (k) performs scanning from an infinite range to the nearest range. The incident rays reflected by the fixed mirror (a) are imaged at the photoelectric cell (e), while the incident rays reflected by the movable mirror is imaged at the photoelectric cell (f). Those images are compared by a coincide detecting module (g) and when the output of the photoelectric cells (e) and (f) coincided, an electromagnet (h) connected to an output circuit is demagnetized so that an armature lever (i) is released to engage with a retaining section (m) of the lens tube (k) stopping the movement of the lense tube (k) at the in-focus position. A problem arises when focus detection is impossible. In that case the electromagnet is kept magnetized so that the lens tube (k) is allowed to move as far as the extremity of its stroke where the photographing mechanism is actuated with the lense set in focus of the nearest range. The elements (Z) and (Y) applicable to the present invention are shown by dotted lines to indicate that they are not part of the conventional automatic focusing mechanism.
An object of the present invention is to provide an automatic focusing device having a system which checks the operation of the shutter or a system which send warning signals when focus detection is impossible.
Another object of the invention is to improve the accuracy of focus detection.